Assault vest Tyranitar / physically defensive mega-Venusaur is a pairing ive been thinking about
Ttar struggles the most against heavy hitting physical attackers who eat its stab moves, such as terrakion, conkeldurr, landorus-therian, etc. Its biggest weaknesses are fighting and water which venusaur resist quite nicely. Mega-Venusaur is very bulky mon with a lot of utility options and with his ability is only weak to psychic, and flying, both of which tyranitar resist handily. These resistances not only seem nice on paper, but in practice they have some merit. COvering weaknesses is nice, but what good is covering a weakness to for example ice, when the pokemon that typically runs an ice move, also runs, for example a strong ground move that will KO your ice resistance. In this case mega-venusaur's psychic weakness blends perfectly with assault vest tyranitar. A large majority of pokemon that use psychic moves are special attackers, almost all are psychic types that can be pursuit trapped by tyranitar. Even a typical focus blast from a psychic type like alakzam, do not even 2HKO without a boosting item, whereas tyranitar's pursuit is going to hurt that same pokemon A LOT. Venusaur's fighting resistance goes well with tyranitar, most fighting types carry ghost, dark, ice and rock moves for coverage, none of which mega-venusaur is weak to. This coupled with mega venusaur's ability to roar out set up sweepers, heal with synthesis, leech seed, giga drain, deal real damage with sludge bomb, EQ or power whip and put things to sleep with sleep powder make venusaur no defensive or offensive/supporting slouch. I guess some mons like tornadus who run acrobatics/super power can pick apart this core, and mega-venusaur's lack of leftovers/good healing in the sand make this duo struggle a bit. Some other points are that mega venusaur handily resists fairy attacks which t-tar is weak to, in fact, special attacking flying or fairy types with fighting moves like togekiss' aura sphere, are going to find themselves on the wrong end of a stone edge if they think their fighting move will KO. Another point is that one of the special attackers that beats assault vest t-tar into the dirt, keldeo, whose fighting STAB targets's his defense is hopeless against venusaur. Hydro pump isnt geting through that special defense, sacred sword isnt doing anything, venusaur isnt even weak to icy wind, but strikes back with roar against a sub CM, and giga drain/power whip against any attacker.
not perfect, but I've tried it and both mons have pretty good utility.